Ah, sorry, my mistake. The option is only available under the latest version of Windows 10.
The upgrade is free but can be a bit of a pain if you're up for getting it from the installation media ( https://www.microsoft.com/en-gb/softwar ... /windows10
I can't quite remember every step I took but it was something like;
* Remove every app from the tablet, freeing up as much space as possible (I may have done a reset for this)
* Run the installation media creation tool to a file
* Copy the contents of the downloaded ISO file (after mounting it) into a folder on the C: drive
* Remove the SD card (this bit is important)
* Run the upgrade from the folder into which you placed the downloaded files
* Wait a long while
* Job's a good one
I can't promise that this is everything I did, but I'm sure it was along these lines.
Regarding the HD, if you're referring to internal memory, after the OS and some of the Office apps, 10GB sounds about right for what's left (but Windows 10 frees some of that up as well).
Good luck. I find this tablet a bit of work to keep going, but for the price it really is pretty fantastic.